1. Field of the Invention
This application relates to a method of forming diffractive features in a layer of material and more particularly to the formation of diffractive optical elements (DOEs), such as for example diffraction gratings, diffusers, volume holograms, and holographic optical elements, which employ such diffractive features.
2. Description of the Related Art
In recent years, applications for diffractive optical elements and holograms has increased. DOEs and holograms are used for decoration and signs, as illustrations in publications such as magazines, to provide protection against tampering or counterfeiting as well as for beam shaping; the later being the function of holographic optical elements. Holograms, and more generally DOEs comprise a plurality of diffractive features arranged such that an input light beam directed onto the DOE or hologram is transformed by diffraction into an output beam forming a predefined image or having a particular shape. For reflective structures, the output beam corresponds to the reflection of the input beam; similarly for a transmissive structures, the output is produced by transmitting the input beam through the hologram or DOE. In either case, the input beam is diffracted to form the output beam.
Of significant importance, is the efficiency of the diffractive optical element or hologram, that is the intensity of the output beam in comparison to the input beam from which it originates. Efficient DOEs produce brighter output beams for a given intensity input. Related to efficiency is the angular selectivity. For some holograms, for example, a single output beam results. Other holograms however produce a number of output beams each directed at a different angle with respect to the input beam. For holograms employed for image formation, proper viewing involves looking at the hologram from a location so as to receive the output beam at the eye. Accordingly, in holograms that are highly angularly selective, an image can only be viewed from a particular narrow range of angles. An additional consideration, the ease of fabricating and thus the cost of the hologram, often determines whether a hologram is practical for a particular application. How a hologram is produced, and the effort that is involved, depends on the type of hologram. Surface holograms include a surface having a predetermined undulating topography defining features that diffracts light in an intended way. Manufacture of the surface hologram involves formation of this patterned surface, which can be accomplished by pressing a surface relief master tool into a soft film to produce the appropriate topographic relief pattern on the surface of the hologram. This process, conventionally referred to as embossing, is relatively simple and inexpensive compared to the manufacturing process involved in creating volume holograms. Volume holograms comprise a layer of material having a predetermined pattern of refractive index variations defining diffractive features within the medium that diffract an incoming beam in a desired manner. Recording of volume holograms conventionally involves exposing a photosensitive material to a laser beam; such arrangements, however, are particularly sensitive to vibration and air currents as well as fluctuations in temperature. Although volume holograms are more difficult to fabricate than surface holograms, they have a higher optical efficiency, i.e., increased wavelength selectivity, and thus are often more desirable than surface holograms.
What is needed is diffractive optical elements and holograms that provide high optical efficiency and that are easy to manufacture.